The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography. For instance, patterns can be formed from a photo resist (PR) layer by passing light energy through a mask (or reticle) having an arrangement to image the desired pattern onto the PR layer. As a result, the pattern is transferred to the PR layer. In areas where the PR is sufficiently exposed and after a development cycle, the PR material can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, etc.). Portions of the PR layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer. The exposed portions of the underlying layer can then be etched (e.g., by using a chemical wet etch or a dry reactive ion etch (RIE)) such that the pattern formed from the PR layer is transferred to the underlying layer. Alternatively, the PR layer can be used to block dopant implantation into the protected portions of the underlying layer or to retard reaction of the protected portions of the underlying layer. Thereafter, the remaining portions of the PR layer can be stripped.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. As a result, there is a corresponding need to increase the resolution capability of lithography systems. One promising alternative to conventional optical lithography is a next-generation lithographic technique known as immersion lithography. In immersion lithography, the wafer to imaged by a lithography system is placed in a liquid medium, through which the patterned light is transmitted. The immersion medium replaces an air gap that is conventionally present between the final lens of a conventional dry lithography imaging system and the wafer.
However, attempts to implement immersion lithography have encountered a number of challenges. For example, minor variations in the index of refraction of the immersion medium can adversely effect the quality of the exposure pattern incident on the wafer.
Accordingly, there exists a need in the art for improved immersion lithography systems and associated methods of controlling imaging using immersion lithography systems.